Recently, the development of information processing system machines using an electrophotographic process is remarkable, and in particular, laser printers and digital copying machines transferring information to digital signals and performing information recording by means of light are significantly improved in regard to print quality and reliability. For these rapidly spread laser printers and digital copying machines, In future, further speeding up or miniaturization as well as the attainment of high image quality will be required. Furthermore, recently, the demands of a full-color laser printer and a full-color digital copying machine capable of full-color printing are rapidly increasing. Particularly, in the case of performing full-color printing, the speeding up and the miniaturization of the apparatuses are further important problems since overlapping of toner images with at laser four colors is required.
In order to realize the speeding up and the miniaturization of the apparatuses, it is necessary to improve the sensitivity of an electrophotographic photoconductor used for them and to reduce the diameter of the photoconductor. Particularly, since at least four photoconductors are included in the apparatus in the case of a tandem type effective for satisfying both of the attainment of full-color and the speeding up, the degree of the request for the reduction in the diameters of the photoconductors is much high. However, with the progression of the reduction in the diameter of the photoconductor, the exchange frequency of a conventional photoconductor becomes much frequent, which is further serious problem, particularly, in a high-speed machine, since the photoconductor is be used in a severe condition. Therefore, not only the attainment of high sensitivity of a used photoconductor but also the attainment of very high durability thereof is essential for realizing the speeding up and the miniaturization of the apparatus.
The attainment of high sensitivity of a photoconductor necessary for responding to the speeding up of the apparatus, a charge generation material with large quantum efficiency is essential. In an organic highly-sensitive photoconductor, titanyl phthalocyanine having at least a maximum diffraction peak at 27.2° in regard to a diffraction peak (±0.2°) at a Bragg angle in a XRD (CuKα line (wavelength of 1.542 Å)) is widely used for and very effective as a charge generation material. However, there is anxiety such that background contamination significantly occurs in the photoconductor using titanyl phthalocyanine because of the deterioration of charging caused by aggregation or fatigue of the pigment. The background contamination is a phenomenon such that printing is made in a white base region in which no image should be printed normally and small black spots innumerably are generated, and has a characteristic of increasing the influence thereof by the repeated use even though it is not problematic at an initial state. Thus, the background contamination is a large factor for determining the life of a photoconductor, and since the speeding up of the apparatus can be attained by the photoconductor using titanyl phthalocyanine but the influence of background contamination becomes larger, image quality stability is poor and the satisfaction of both the speeding up and the attainment of high durability has not been realized. Therefore, in regard to the conventional photoconductor using titanyl phthalocyanine, the exchange frequency of the photoconductor is significantly large and providing stable mages over a long term has not been realized, as it is used in a high-speed machine.
For the attainment of high durability of a photoconductor, it is necessary to enhance the stability of image quality and, particularly, to suppress the generation of background contamination. For the mechanism of the generation of background contamination, the origin thereof is considered that charge with a polarity opposite to that induced at the side of an electrically conductive support leaks locally and is injected to a photoconductive layer and, further, to the surface of the photoconductor, and such portion is easy to be developed. Then, even though the background contamination on the photoconductor is not made apparent at the beginning, the background contamination is made apparent by the fatigue (deterioration of charging) of the photoconductor caused by repeated use or the increase of electrical field strength caused by wearing of the photoconductive layer, and, therefore, is a major factor for determining the life of the photoconductor.
In regard to the electrostatic fatigue of a photoconductor, the fatigue of the photoconductor progresses by repeatedly performing charging or light exposure in image formations and the lowering of a charging electric potential or the elevation of an electric potential (having the same meaning as a residual electric potential) at an exposure portion caused thereby causes the degradation of image quality. Particularly, the lowering of the charging electric potential further increases the influence of the charge leak from an electrically conductive support and facilitates to make the background contamination be apparent. In regard to the wearing of a photoconductor, as a surface layer of the photoconductor is worn away by the friction with a cleaning member, etc., and, thereby, the film thickness of the photoconductor surface layer, the degradation of image quality is caused by the elevation of electrical field strength, the increase of scratches on the photoconductor surface, etc. Particularly, as the electrical field strength elevates by the decrease of the film thickness, the generation of background contamination significantly increases.
Thus, in order to realize the speeding up, miniaturization, and long life of an image formation apparatus, it is apparent that the attainment of high sensitivity, small diameter, and high durability of an electrophotographic photoconductor used therein are needed, but the major problem on realizing them is the suppression of background contamination as mentioned above. As mentioned above, a matter originating from charge injection from an electrically conductive support, a matter originating from aggregation and coarse particles or crystallographic type of an charge generation material in a charge generation layer, and a matter originating from the increase of electrical field strength by the wearing of a layer formed on a charge transportation layer or the top surface of a photoconductor are provided as the causes of this background contamination.
In order to realize the high durability of a photoconductor, it is necessary to suppress the background contamination and, to this end, it is important to suppress the charge injection from an electrically conductive support. As a conventional technique in regard to the charge injection from an electrically conductive support being one of the causes of the generation of background contamination, a technique for providing an underlying layer or an intermediate layer between the electrically conductive support and the photoconductive layer has been suggested.
For example, an intermediate layer of cellulose nitrate-type resin, an intermediate layer of nylon-type resin, an intermediate layer of maleic acid-based resin, and an intermediate layer of polyvinyl alcohol resin are disclosed in Japanese Laid-Open Patent Application Nos. 47-6341, 60-66258, 52-10138, and 58-105155, respectively.
However, since the electric resistances of these single-layered and singularly resinous intermediate layers are high, the elevation of a residual electric potential is caused and the lowering of image concentration or the deterioration of a gradation characteristic is caused in negative or positive development. Also, since these intermediate layers indicate ionic conduction property originating from impurities, etc., the tendency that the electric resistance of the intermediate layer becomes notably high under low temperature and low humidity, whereby the residual electric potential significantly elevates, and the electric resistance of the intermediate layer is lowered under high temperature and high humidity, whereby the background contamination is easy to occur, has been found. Therefore, it is necessary to make the intermediate layer be a thin film and the actual situation is that the sufficient suppression of background contamination has not been realized.
For solving these problems, an approach of dispersing an electrically conductive additive in an intermediate layer has been suggested as a technique for controlling the electric resistance of the intermediate layer.
For example, an intermediate layer in which a carbon or a chalcogen-type material is dispersed in a curable resin, an intermediate layer of a thermally polymerized material for which a quaternary ammonium salt is added and an isocyanate-type curing agent is employed, a resinous intermediate layer to which an electrical resistance controlling agent is added, and a resinous intermediate layer to which an organometallic compound is added, are disclosed in Japanese Laid-Open Patent Application Nos. 51-65942, 52-82238, 5-210259, and 58-93062, respectively. However, in a single one of these resinous intermediate layers, the tendency that the background contamination increases is found even though the reduction of a residual electric potential is realized, and there are problems that interference fringes of light appear on an image and Moire image generates, in a recent image formation apparatus using coherent light such as laser light.
For the purpose of controlling the electrical resistance of an intermediate layer while the generation of Moire is suppressed, a photoconductor with an intermediate layer that contains filler has been suggested.
For example, a resinous intermediate layer in which an oxide of aluminum or tin is dispersed, a resinous intermediate layer in which electrically conductive particles are dispersed, an intermediate layer in which a magnetite is dispersed, and a resinous intermediate layer in which titanium oxide and tin oxide are dispersed, are disclosed in Japanese Laid-open Patent Application Nos. 58-58556, 60-111255, 59-17557, and 60-32054, respectively, and a resinous intermediate layer in which powders of a boride, nitride, fluoride, or oxide of calcium, magnesium, aluminum, etc. are dispersed, are disclosed in Japanese Laid-Open Patent Application Nos. 64-68762, 64-68763, 64-73352, 64-73353, 1-118848, and 1-118849.
Although such an intermediate layer in which filers are dispersed is effective for the suppression of Moire, it is necessary to increase the filler content for reducing the residual electric potential and it is preferable to reduce the quantity of the filler for suppressing the background contamination, but balancing of the background contamination and the residual electric potential has not been achieved yet. Further, as the content of the resin becomes lower, the adhesion property with the electrically conductive support lowers and the problem arises that peeling between the support and the intermediate layer is easy to occur, and thus there are many problems for the attainment of high durability of the photoconductor. Particularly, the influence has been critical in a belt-shaped photoconductor in which an electrically conductive support is flexible.
To solve such problems, an approach of stacking of intermediate layers is suggested. The stacking configuration is generally classified into two types, that is, one of them is a stack s such that a resinous layer in which filler is dispersed and a resinous layer that contains no filler are stacked on an electrically conductive support in order (see FIG. 1), and the other is a stack such that a resinous layer that contains no filler and a resinous layer in which filler is dispersed are stacked on an electrically conductive support in order (see FIG. 2).
As the former configuration is described in detail, on an electrically conductive support 10, an electrically conductive filler dispersion layer 21 in which low-resistant filler is dispersed is provided, the resinous layer 22 is stacked thereon, and then a photoconductive layer 30 is provided thereon, for covering up the defect of the electrically conductive support 10. These are disclosed in, for example, Japanese Laid-Open Patent Application Nos. 58-95351, 59-93453, 4-170552, 6-208238, 6-222600, 8-184979, 9-43886, 9-190005, and 9-288367. This configuration is capable of preventing the generation of Moire by the filler dispersion layer 21 that contains electrically conductive filler and can also obtain an effect of suppressing the background contamination because of having the resinous layer 22 thereon, but film thickening and film thinning will cause remarkable elevation of the residual electric potential and the increase of background contamination, respectively, and no sufficient satisfaction is obtained on realizing of the balance therebetween, since only the resinous layer 22 suppresses carrier injection from the electrically conductive support 10, similar to the case of using the aforementioned resinous layer singularly. Also, since the insulating resinous layer 22 is stacked on the filler dispersion layer 21 and it is necessary to thicken the film thickness of the filler dispersion layer 21 (equal to or greater than 10 μm) for covering up a defect of the electrically conductive support 10, it is difficult to suppress the background contamination by increasing the resistance of filler contained in the filler dispersion layer 21 because the influence of a residual electric potential significantly increases.
Also, a photoconductor in which an electrically conductive layer, an intermediate layer, and a photoconductive layer that contains titanyl phthalocyanine crystal are stacked is disclosed in Japanese Laid-Open Patent Application Nos. 5-100461, 5-210260, and 7-271072. However, it is difficult to sufficiently suppress the influence of background contamination by only stacking the electrically conductive layer and an intermediate layer. In addition to the aforementioned reason, it is because a factor of the background contamination is also included in titanyl phthalocyanine used for the photoconductor.
On the other hand, the latter configuration is a configuration such that a resinous single layer 22 for suppressing carrier injection is provided on an electrically conductive support 10, a filler dispersion layer 21 that contains filler is provided thereon, and a photoconductive layer 30 is provided thereon. This is disclosed in, for example, Japanese Laid-Open Patent Application Nos. 5-80572 and 6-19174. In this configuration, since carrier injection is suppressed by the resinous layer 22 and the filler dispersion layer 21 that contains filler and is stacked thereon has an influence given on the residual electric potential even if electrically conductive filler is not particularly contained, the suppression effect of the carrier injection is enhanced and the efficiency on the balancing between the residual electric potential and the background contamination is higher than that of the former configuration.
Thus, although a configuration such that plural underlying layers 21, 22 are stacked for function separation indicates high efficiency on satisfying the prevention of Moire, the suppression of background contamination, and further the reduction of a residual electric potential, it is necessary to thin the film of the resinous layer 22 for use and the tendency that the humidity dependence of the background contamination and the residual electric potential is large and the film thickness dependence thereof becomes large is found dependent on resin used for it, whereby it does not necessary has high stability.
Also, many investigations are made for resin used for an underlying layer and polyamide is comparatively frequently used. Among these, an approach of using an underlying layer or intermediate layer that contains N-alkoxy(methoxy)methylated nylon is disclosed. For example, an approach of containing an alkoxymethylated copolymeric nylon with the degree of alkoxymethylation of 5 through 30% in an underlying layer and an approach of containing an inorganic pigment and a cross-linked N-alkoxymethylated polyamide as a binder resin in an intermediate are disclosed in Japanese Laid-Open Patent Application Nos. 9-265202 and 2002-107984, respectively, and an approach of an underlying layer that is made of an N-alkoxymethylated nylon resin wherein each of the elemental concentrations of impurities, that is, Na, Ca, and P atoms contained in the resin is equal to or less than 10 ppm, an approach of containing an N-alkoxymethylated polyamide copolymer based on λ-amino-n-lauric acid in an intermediate layer, and an approach of containing a polyamide resin having a unit component having a certain structure in an intermediate layer, are disclosed in Japanese Patent Nos. 2718044, 3086965, and 3226110, respectively.
Thus, the approaches of stacking an underlying layer or an intermediate layer and containing N-alkoxymethylated nylon in an underlying layer or intermediate layer are publicly known, which are effective as means for suppressing the charge injection from the electrically conductive support and enhancing the background contamination suppression effect.
However, only stacking of the plural underlying layers or containing N-alkoxymethylated nylon in the underlying layer or intermediate layer is effective to the suppression of background contamination caused by the charge injection from the electrically conductive support but the influence to the residual electric potential, the Moire, the background contamination, and environmental dependence of a residual electric potential, etc. is not small. Particularly, as the electric field strength increases with the progression of photoconductor wearing, the background contamination significantly increases and the enhancement of a charge blocking function of the underlying layer or intermediate layer also under high electric field strength causes remarkable elevation of the residual electric potential, and, therefore, the suppression effect to the background contamination in repeated use of the photoconductor is not sufficient by only suppressing the charge injection from the electrically conductive support by means of the underlying layer or the intermediate layer and the high durability of the photoconductor has not been realized yet.
Furthermore, when coating is applied on the underlying layer, it is necessary to be insoluble to solvent thereof, and a defect of the coating film influences the background contamination, and, therefore, high quality of coating film and high coating property such that the deviation of film thickness is small even in a thin film are required.
Moreover, the cause of background contamination is not only charge (hole) injection from an electrically conductive support to a photoconductive layer but also cannot ignore the influence on the photoconductor as mentioned above. Particularly, the cohesiveness of conventional titanyl phthalocyanine is strong and, when it is used for a charge generation layer, the deterioration of charging and the increase of dark decay in a local portion in which agglomerates or coarse particles exist and the background contamination is made apparent even though the charge injection from an underlying layer is suppressed. Also, since the purity of titanyl phthalocyanine has a large influence, the charging deterioration is significantly caused by the containment of impurities and the increase of dark decay is caused by fatigue, whereby the resistance to the background contamination is significantly deteriorated.
Thus, in the conventional techniques, the elevation of a residual electric potential or the environmental dependence increases significantly as the background contamination is suppressed, while the background contamination suppression effect becomes insufficient as the elevation of a residual electric potential is suppressed, etc., the balancing therebetween has not been realized. Thus, the background contamination includes not only the influence of the charge injection from an electrically conductive support but also many factors such as the influence of coarse particles or impurities of titanyl phthalocyanine contained in a photoconductive layer or charge generation layer, and, however, besides them, the increase of the electric field strength by the reduction of film thickness of the photoconductor is important as a factor giving a large influence to the background contamination.
Therefore, a device to enhance the wear resistance has been made for a charge transportation layer or a protection layer formed on the top surface of a photoconductor. As conventional techniques for improving the wear resistance of a photoconductive layer, (i) the use of a curable binder for a crosslinked-type charge transportation layer (ex. see Japanese Laid-Open Patent Application No. 56-48637), (ii) the use of a polymeric charge transportation material (ex. see Japanese Laid-Open Patent Application No. 64-1728), and (iii) the dispersion of inorganic filler in a crosslinked-type charge transportation layer (ex. see Japanese Laid-Open Patent Application No. 4-281461) can be provided.
Thus, since the change of electric field over time can be made small by enhancing the wear resistance of a photoconductor, high effect for the suppression of back ground contamination can be obtained.
However, among these techniques, (i) the use of a curable binder has a tendency of facilitating to elevate the residual electric potential and to cause the lowering of image concentration by impurities such as a polymerization initiator and an unreacted residue.
Also, (ii) the use of a polymeric charge transportation material can improve the wear resistance to some extent, but has not led to sufficiently satisfy the durability required for an organic photoconductor. Also, in regard to the polymeric charge transportation material, since the polymerization and purification of the material are difficult and it is difficult to obtain high purity, the electric characteristics of the material is difficult to be stable. Furthermore, a problem on manufacture such that coating liquid becomes highly viscous may occur.
(iii) The dispersion of inorganic filler exerts high wear resistance compared to a photoconductor in which a normal low-molecular-weight charge transportation material is dispersed in an inactive polymeric molecules but there is a tendency of facilitating to elevate a residual electric potential by a charge trap existing on an inorganic filler surface and to cause the lowering of image density. Also, when the irregularities made of inorganic filler and a binder resin on a photoconductor surface is large, improper cleaning occurs, which may cause toner filming or image deletion.
These techniques (i), (ii), and (iii) may be effective for the suppression of background contamination but have a disadvantage on a residual electric potential, a cleaning property, etc., and has not led to satisfying the durability sufficiently because of the influence of an image defect caused by it.
Furthermore, a photoconductor that contains a substance obtained by curing multi-functional acrylate monomers for improving the wear resistance and damage resistance of (i) is known (ex. see Japanese Patent No. 3262488). However, in regard to this photoconductor, there is a description of the meaning of containing this substance obtained by curing multi-functional acrylate monomers in a protection layer provided on a photoconductive layer but there is an only description that a charge transportation material may be contained in this protection layer and no specific description, and further in the case of simply containing low-molecular-weight charge transportation material in a crosslinked-type charge transportation layer, there is a problem of compatibility with the aforementioned cured substance, whereby the precipitation or white turbidity phenomenon of a low-molecular-weight charge transportation material occurs and not only image concentration lowered by the elevation of an electrical potential of exposed portion but also the mechanical strength might lower. Furthermore, since, specifically, the monomers are reacted on the condition of containing a polymeric binder in regard to this photoconductor, three-dimensional network structure develops insufficiently and the crosslink density thereof is subtle, thereby having not led to be able to exert a drastic wear resistance.
As a wear resistance technique for a photoconductor replacing them, it is known that a charge transportation layer formed by using coating liquid made from monomers having a carbon-carbon double bond, a charge transportation material having a carbon-carbon double bond, and a binder resin is provided (ex. See Japanese Patent No. 3194392). It is considered that this binder resin serves to improve the adhesion property of a charge generation layer and a crosslinked-type charge transportation layer and, further, to relax the internal stress of a film at the time of curing a thick film, and is generally classified to one having a carbon-carbon double bond and having reactivity to the aforementioned charge transportation material and one having no double bond mentioned above and having no reactivity.
Although this photoconductor satisfies both a wear resistance and a good electrical characteristic and draws attention, the compatibility of a binder resin and a cured substance produced by the reaction of the aforementioned monomer and the charge transportation material is low and layer peeling occurs in a crosslinked-type charge transportation layer, thereby causing damage or the fixation of an external additive and paper powder in toner, in the case of using one having no reactivity as the binder resin. Also, as mentioned above, three-dimensional network structure develops insufficiently and the crosslink density thereof is subtle, thereby having not led to exert a drastic wear resistance. In addition, the specifically described monomer used in this photoconductor is two-functional and the wear resistance thereof has not led to satisfaction yet in these respects. Also, in the case of using a binder resin having reactivity, the molecular weight of the cured substance increases but the number of intermolecular crosslinkages is low, and it is difficult to satisfy both the bonding quantity and the crosslink density of the aforementioned charge transportation material and the electrical characteristic and the wear resistance were insufficient.
Also, a photoconductor that contains a compound for which a hole transportation compound having two or more chain-polymerizable functional groups in the same molecule is cured is known (ex See Japanese Laid-Open patent Application No. 2000-66425). Although the crosslink density of this photoconductive layer can be high, it has a high hardness, but distortion occurs in a cured substance and the internal stress thereof becomes large since a bulky hole transportation compound has two or more chain-polymerizable functional groups, whereby a crack or peeling in the cross-linked surface layer may be easy to occur in use over a long term. It is considered that the photoconductor having a crosslinked photoconductive layer for which charge transporting structures are chemically bonded in these conventional techniques has no sufficient overall characteristics in the present circumstances.
Thus, while many conventional techniques for enhancing the wear resistance of a photoconductor are disclosed and it is known that they are effective for the suppression of background contamination, the generation of an image defect such as the deterioration of resolution or gradation, image deletion, and the adhesion of foreign substances is induced even if the background contamination can be suppressed, that is, the elevation of a residual electric potential becomes easy to occur, image deletion becomes easy to be caused dependent on humidity, a photoconductor surface becomes difficult to be refaced whereby filming becomes easy to occur, etc., and the stability of image quality is greatly deteriorated, and, therefore, in fact, the attainment of high durability of a photoconductor has not been realized.
Also, only enhancing the wear resistance of a photoconductor can reduce the influence to electric field with time but the influence caused by charge injection from an electrically conductive support is facilitated by the deterioration of charging caused by the fatigue of the photoconductor, and, therefore, the effect is not sufficient. Therefore, in order to suppress the generation of background contamination by repeated use of a photoconductor and to realize the attainment of high durability of the photoconductor, it is necessary to suppress the charge injection from an electrically conductive support and to reduce the wearing of photoconductor caused by repeated use.
In order to completely suppress the generation of background contamination, it is not achieved unless not only the charge injection from an electrically conductive support is suppressed but also the reduction of film thickness of a photoconductive layer caused by repeated use is suppressed. Also, the attainment of high durability of a photoconductor can be achieved only by suppressing the aforementioned generation of background contamination, making the stability of a residual electric potential over time be high, and reducing the degradation of image quality as side effect such as filming, the adhesion of foreign substances, and image deletion caused by enhancing the wear resistance, so that high image quality can be maintained over a long term even in repeated use. However, the satisfaction of all of them has not been realized in the conventional techniques and the actual circumstance is that the high durability of a photoconductor has not been achieved yet.